Water and Energy Waste at Thermal Power Plants
The thermal efficiency of a modern steam power plant is only ˜35%. Most of the energy in its fuel is wasted. Inefficiency is principally due to heat rejection in the cooling tower, where waste heat from the steam turbine exhaust is dumped into the atmosphere as latent heat in vapor of cooling water.
The vapor out of the cooling tower is wasted water as well as wasted energy. Fresh water used for thermal power plant cooling water is becoming a precious commodity, forcing a choice between water for power and water for people. The amount of water wasted by conventional thermal power plants is enormous. The United States Geological Survey (USGS) estimates that thermoelectric power generation requires 3.6×1010 cubic meters (m3), or 136 billion gallons, of fresh water per day. In the year 2000, that was 39% of freshwater withdrawals in the United States, slightly less than agricultural irrigation (40%), and much more than other industrial and residential use.
A need exists for improved means for condensing exhaust steam, avoiding the water and energy waste of cooling towers and conventional steam condensing. An object of the present invention is to fill that need.
Turbine Exhaust Steam
Power plant turbine exhaust steam is wet, i.e. it has a high weight percentage of condensate. Turbine blade erosion concerns place a lower limit on quality (weight percent of vapor) of 0.88, with most turbines operating in the 0.90-0.95 range. Exhaust steam still has high energy content, or enthalpy (kJ/kg), even after doing work in the turbine, but its energy is principally in latent heat of condensation (hfg). The latent heat must be extracted so that the water can condense and be pumped back into the boiler to be re-used in the Rankine cycle.
Mass flow through a steam turbine is pushed by the high pressure of the boiler and simultaneously pulled by the low pressure of a steam condenser. Condensation of vapor in the steam condenser creates a vacuum (typically 0.03-0.4 bar) which pulls more steam through the turbine.
The conventional steam condenser (surface condenser) comprises a shell and tubes disposed within the shell. The tubes are part of a cooling water circuit. Turbine exhaust steam is injected into the shell, and cooling water circulating through the tubes bears off the waste heat to the cooling tower. The condensate drips into a hotwell and is pumped back into the boiler. The cooling water is sprayed into a cooling tower, where evaporative cooling rejects the turbine exhaust waste heat into the atmosphere. Vapor out of the cooling tower is wasted water. The water volume in the cooling water circuit must be replenished by make-up water, which must be carefully pre-treated to prevent scaling and biofouling within the tubes.
Re-use of the cooling water and its continuous evaporation concentrates the dissolved solids, so periodically some blow-down is discharged to purge the system. Evaporation builds up a high concentration of limestone (calcium carbonate, CaCO3), sulfates, and other scale-forming compounds in the cooling water. Scale is a tough and insulating crust which is precipitated by heat on the interior walls of the tubes. The blow-down has a high percentage of total dissolved solids and is a water pollution problem as well as a waste of a precious resource.
A steam ejector communicating with the shell purges any noncondensible gases and also helps to maintain a very low pressure in the shell. Low pressure in the condenser is key to optimal Rankine cycle efficiency.
Cooling Towers
The waste heat absorbed by the cooling water of the shell and tube surface condenser could be discharged immediately by dumping the cooling water into the environment (the once-through process), but this option is not favored because thermal pollution of the environment is usually not acceptable. Air cooling is another option, but for large power plants it is not satisfactory because of the low heat flux between fins and ambient air, even when the air is blown. When the heat load is large and the ambient air is hot, such as on a hot summer day when many air conditioners are running, air cooling may fail.
The preferred method for reliable heat rejection is to extract the heat load by evaporative cooling in order that the cooling water can be recycled through the tubes. The conventional evaporative cooling method involves a cooling tower. Within the cooling tower, an updraft of air meets a spray of hot cooling water, and evaporation cools the spray. Typically 3-6% of cooling water sprayed in is lost by evaporation in the cooling tower, a large waste of water as well as energy. In a typical 700 MW coal-fired power plant, having a circulation rate of 71,600 m3/hr, the water waste is 3,600 cubic meters an hour.
Nuclear and gas plants also waste water in heat rejection from their steam turbine exhaust, no less than coal plants. A major siting constraint on nuclear plants is the scarcity of fresh water. Of course, seawater or alkaline water won't work for a cooling water circuit because it contains scale-forming dissolved solids which precipitate at high temperatures and would quickly clog the tubes.
Petroleum refineries have very large cooling water systems. A typical large refinery processing 40,000 metric tons of crude oil per day (˜300,000 barrels per day) circulates about 80,000 cubic meters of water per hour through its cooling tower system, evaporating and wasting a prodigious amount of precious fresh water. Dumping vapor in the atmosphere is not a sustainable practice, and a need exists for an alternative method for heat rejection which does not waste water.
Another reason, besides water waste, to eliminate cooling towers is the danger they pose to public health. The warm, moist environment in a cooling tower provides a favorable habitat for the Legionella bacteria that cause Legionellosis, a type of pneumonia commonly known as Legionnaire's disease. Studies have shown that 40 to 60% of cooling towers are infected with Legionella. Entrained infected mist droplets in the drift out of the stack provide transportation for these bacteria to contact with humans kilometers away. Each year in the United States, 8,000-18,000 people are infected. Therefore biocideal treatment is necessary, and there is strict regulatory scrutiny.
Infected steam billowing from cooling towers is a visible threat to the health of the community. Public acceptance of the presence of power plants is an important consideration in siting. Cooling towers, whose profile is associated with the nuclear disaster at Three Mile Island, and which emit huge volumes of what looks like smoke, are not good for public relations. They are a prominent and objectionable feature of any power plant. Now that fresh water has become a scarce resource, coal, gas, and nuclear power plants have a need for an alternative to cooling towers, and the present invention is intended to fill that need.
The Ranque-Hilsch Vortex Tube
The vortex tube is an axial counterflow device having no moving parts, wherein feed pressure drives thermal separation into a cold stream and a hot stream. See Ranque, U.S. Pat. No. 1,952,281 (1934). Length of a vortex tube is typically between 30-50 tube diameters. How thermal separation occurs in a vortex tube has not been settled, and interesting speculation abounds. See Chengming Gao, Experimental Study on the Ranque-Hilsch Vortex Tube (Eindhoven 2005) http://alexandriatue.nl/extra2/200513271.pdf.
In operation, a tangential feed nozzle at a cold end of the vortex tube jets in a pressurized gas feed which swirls along the tube to a conical impedance partially blocking the opposite end, the hot end. The conical impedance is a valve pointing toward the cold end, and there is a passage around the conical impedance where the hot stream exits at a higher temperature and lower pressure than the feed. A cold stream rebounds from the conical impedance in an axial jet inside the feed vortex and exits the cold end at a lower temperature and lower pressure than the feed. Thus a hot stream and a cold stream are separated from a feed stream, both at lower pressure. Feed pressure drives thermal separation in a very simple and easily scalable device. Commercial applications of the vortex tube include spot cooling for welding and machining operations.
Cascading of vortex tubes has the problem of reduced feed pressure at each successive stage of the cascade, with consequent loss of separation, unless there is some boosting of feed pressure between stages. The present invention provides means for inter-stage boosting in multiscale cascades of vortex tubes.
Some investigation of application of the vortex tube to steam condensers has been done. Schwieger U.S. Pat. No. 6,516,617 (2003) discloses a system which uses a cascade of static vortex tubes to separate exhaust steam, each stage of the cascade producing a hot stream and a cold stream. In the Schwieger system, the cold stream carries off condensate. At each stage, condensate in the cold stream is pressurized by a pump and then is heated by the hot stream, becoming feed for a secondary steam turbine. However, in field testing of natural gas separation, condensate was found in the hot stream, and not the cold stream. K. Hellyar “Gas Liquefaction using a Ranque-Hilsch Vortex Tube: Design Criteria and Bibliography” (MIT 1979) http://dspace.mit.edu/bitstream/handle/1721.1/16105/07771761.pdf at p. 16. This experimental result makes sense because condensate is much denser than the gas, so it will be centrifugated out in the vortex and will be extracted from the vortex tube along with the hot stream. The present invention, in accord with this experimental result, teaches away from the cold stream advection of condensate disclosed in Schweiger.
Nicodemus, U.S. Pat. No. 4,037,414 (1977) also uses a vortex tube in a Rankine cycle device wherein the hot stream powers an injector upstream of the boiler, which receives the cold stream and mixes it with the hot stream. Cosby, U.S. Pat. No. 4,479,354 (1984) teaches a vortex tube for scavenging energy in the exhaust steam in order to improve thermal efficiency of a turbine. See also Promvonge, et al., Science Asia 31: 215-223 (2005).